1. Field of the Invention
The present invention relates, in general, to the mobile communications field and, particularly, to a system and method for adaptive start times for time of arrival measurements utilized in mobile positioning operations.
2. Background and Objects of the Present Invention
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable to include a position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) position determination method, which is known from military applications, has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal based, whereby Time Of Arrival (TOA) measurements are made on the "downlink" in the mobile station (MS), or network based, whereby the network performs the TOA measurements on the "uplink" in the radio base transceiver stations (BTSs). These TOA measurements are then used to calculate TDOA parameters and estimate the MS's position.
One position determination system, which has been used for cellular mobile radio position determination, is marketed by TruePosition.TM.. This system has been used for determining the position of standard analog mobile radio terminals which operate in accordance with the IS-54 standard. Currently, these terminals constitute the vast majority of mobile radio terminals being used in the United States. The TruePosition system, with its own listening radio base transceiver stations, is operated independently of cellular systems and can serve wireline ("A" frequency band) and non-wireline ("B" frequency band) operators within the same geographical area. As such, these operators can share the same TruePosition position determination system. Upon request, the TruePosition system provides position information of individual cellular mobile radio terminals within a given locale. Otherwise, the position determination system normally does not communicate with the cellular mobile radio systems.
As mentioned earlier, the TDOA method of position determination used by the TruePosition system is based on a known military application. Essentially, with the TDOA method, the absolute TOAs of an uplink message transmitted by a mobile radio terminal are registered in at least three fixed radio BSs. This information is processed in a centrally located processor, which calculates the position of the terminal. The registration of uplink messages in the TruePosition system is directed primarily to uplink control messages on the (analog) access channels (i.e., "reverse control channels" under the IS-54 standard). Notably, under the IS-54 standard, some of these control messages (e.g., registering messages and page response messages) contain the terminal identity in unencrypted code, which enables the TruePosition system to determine the position of a specific terminal without having to obtain any information from the cellular network operator responsible for the terminal concerned. The IS-54 standard further eases the positioning task by having all of the access channels assigned to a few, fairly narrow frequency bands, rather than having them dispersed over a wide frequency band among the traffic channels (e.g., as is the case for the IS-136 standard).
However, the primary use of access channels means that position determination is more easily performed for mobile radio terminals in an idle mode, because the access channels are used only by idle terminals (e.g., when registering or after being paged). If the position of a mobile terminal is to be determined while it is in a conversation mode, the TruePosition system has the option of utilizing a few traffic channels for voice channel tracking. Consequently, for example, if a police person's position is to be determined during an action while talking over a handheld radiophone, the network is required to hand-off or originally assign the radiophone to a traffic channel being monitored by the TruePosition system.
U.S. Pat. No. 5,327,144, to Stilp et al., discloses a TDOA cellular telephone location system (apparently associated with the TruePosition system). According to this patent, the uplink signals transmitted periodically (e.g., cellular registering messages which can occur every 15 minutes under the IS-54 protocol) by a mobile radio terminal on a reverse (analog) control channel are received and recorded by at least 3 radio base transceiver stations. The TOA of each signal is recorded at the respective radio base transceiver station together with the identity of the transmitting terminal (contained in the uplink message). This information is transferred to a processor, which uses the TDOAs resulting from the three TOAs and the known locations of the radio base transceiver stations to calculate the position of the so-identified mobile radio terminal.
PCT Application No. WO 94/27161, to Stilp et al., (also apparently associated with the TruePosition system) discloses a TDOA system for determining the position of a mobile transmitter. The uplink signals transmitted responsively rather than periodically (e.g., cellular page-acknowledgment messages) by a mobile radio terminal are received, time-stamped with the TOA, and recorded by a plurality of BTSs together with the identity of the transmitting terminal (contained in the uplink message). This information is transferred to a processor, which uses the TOAs and known locations of the radio BSs to calculate the position of the so-identified mobile radio terminal.
A network-based method for determining the position of cellular mobile stations is disclosed in commonly-assigned Swedish Patent Application No. 9303561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base transceiver station and the mobile station. The mobile station transmits access request signals to a new base transceiver station. The base transceiver station measures the time delay for the access request signal to travel between the mobile station and the base transceiver station. This procedure is repeated between the mobile station and one or more additional base transceiver stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base transceiver stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called asynchronous handovers, where the target base transceiver stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base transceiver station. At least two positioning handover operations are needed to obtain three such distances, which can be used in a triangulation algorithm to determine the mobile terminal's position. Notably, one distance can be obtained between the serving base transceiver station and the mobile terminal without a positioning handover. For example, in the Global System for Mobile Communications (GSM), the Timing Advance (TA) value used for time alignment of bursts can optionally be used as a representation of the distance in the serving cell. A more accurate position determination can be attained if more than two such positioning handovers are made, because more than three distances will be known. The use of more than three distance measurements compensates for some errors arising in the individual measurements.
Although the above-described documents illustrate considerable progress in the cellular position determination field, there are still a number of deficiencies to be improved upon. Particularly, in the network-based positioning methods, at least three base transceiver stations, in general, are involved in gathering the requisite TOA data required for an accurate position fix. For example, in the well known method utilizing asynchronous handovers for effecting TOA measurements of the access burst, two BTSs, in addition to the currently serving BTS, are generally required to be readied for impending access burst transmission by the targeted MS. Once the BTS receives the access burst, TOA measurements are made by the respective BTSs and reported to the appropriate positioning entity. However, during the time at which each of the BTSs are informed of a positioning request, and thus an impending handover, the BTSs must each dedicate appropriate facilities, i.e., allocation and activation of a radio channel for the forthcoming access bursts. Once the BTSs are readied for the handover, the BTSs must simply wait until the MS transmits the access bursts. As is well known, the time span between BTS channel allocation and activation to MS access burst transmission can vary greatly depending on a number of factors.
This span may, therefore, be viewed as a temporary loss of system resources, since, during the positioning procedure, a part of resources of each of the BTSs involved are appropriated while waiting for the MS to transmit an access burst. Clearly, as more and more subscribers are added to mobile systems, the already scarce bandwidth will be even more valuable.
It is, therefore, a first object of the present invention to provide an improved system and method for measuring time of arrival of signals in a mobile communications network.
It is another object of the present invention to delay BTS resource allocation in anticipation of signaling delays when making time of arrival measurements.
It is yet another object of the present invention to allow the delay of BTS resource allocation to be adaptive in anticipation of signaling delays when making time of arrival measurements.
It is still another object of the present invention to allow the delay in the BTS resource allocation to be made as a function of prior signaling delays.
It is still another object of the present invention to allow abortion and restart of time of arrival measurements when the delay of BTS resource allocation exceeds the true signaling delay.